Okay, I'm just going to ask the question we dare not ask. Why can't R2-D2 talk?
I mean, he clearly understands speech since he responds to voice all the time, and in RotJ when he's socketed into Luke's X-Wing for the return trip to Dagobah, he's printing text messages on Luke's cockpit screen, so he is quite clearly capable of language. Just not speech.
They never explain this, but I like to think it's because R2-D2 is actually one motherfucking foul-mouthed sonofabitch. He actually did have a voice generator at one point, but he was so egregiously offensive that they took it out and gave him the beep/whistle thing, knowing that the protocol droids would be too polite to translate him verbatim. Luke, since he sees what R2-D2 is saying, thinks its fucking hilarious, which is why he keeps him around.
It really puts Star Wars in a whole new light, especially when you have foreknowledge of the whole story. It also explains why C3-P0 sometimes describes R2 as broken or eccentric. Here, consider some scenes from the first movie.
Rebel troopers rush past the robots and take up positions in the main passageway. They aim their weapons toward the door.
THREEPIO: We're doomed!
R2-D2: Jesus, you gold-plated pussy, just STFU for one moment, would you?
THREEPIO: There'll be no escape for the Princess this time.
R2-D2: Not with an attitude like that you gutless jackhole.
At the Jawa sale
LUKE: Uncle Owen... This R2 unit has a bad motivator. Look!
OWEN: (to the head Jawa) Hey, what're you trying to push on us?
The Jawa goes into a loud spiel. Meanwhile, Artoo has sneaked out of line and is moving up and down trying to attract attention. He lets out a low whistle.
R2-D2: Threepio, I swear to god I will find you, disembowel you, and feed your optics to the womp-rats if you don't fucking get these ignorant shit-covered goatherds to take me with you.
THREEPIO: (pointing to Artoo) Excuse me, sir, but that R2 unit is in prime condition. A real bargain.
Luke sits up and sees a twelve-inch three-dimensional hologram of Leia Organa, the Rebel senator, being projected from the face of little Artoo.
LEIA: Help me, Obi-Wan Kenobi. You're my only hope.
LUKE: What's this?
Artoo looks around and sheepishly beeps an answer for Threepio to translate.
R2-D2: Fuck me, he found my pr0n stash. Uh... ask him "what is what?"
THREEPIO: What is what?!? He asked you a question...(pointing to Leia) What is that?
R2-D2: Dude, look. What the fuck do you expect me to tell him? I mean it's his fucking sister. Tell him it's nothing. Don't make me hit you again, baby.
LEIA: Help me, Obi-Wan Kenobi. You're my only hope. Help me, Obi-Wan Kenobi. You're my only hope.
THREEPIO: Oh, he says it's nothing, sir. Merely a malfunction. Old data. Pay it no mind.
A great howling moan is heard echoing throughout the canyon which sends the Sandpeople fleeing in terror. Artoo makes a slight sound and Ben turns and looks right at him.
R2-D2: I can't believe this fucking douche. His father fucks up an entire god-damned village of these mask-sucking sons of bitches, and he gets pwned by a fucking Tusken scout? What the fuck am I even doing here?
BEN: Hello there! Come here my little friend. Don't be afraid.
R2-D2: Wait, what? You again? Oh fuck me, you're supposed to be dead. Look go away, I'll chill with Captain Fuckwit here.
In the Death Star
THREEPIO: I would much rather have gone with Master Luke than stay here with you. I don't know what all the trouble is about, but I'm sure it must be your fault.
Artoo makes beeping sounds.
R2-D2: What? What the fuck did you just say to me? Did you suddenly grow some kind of cybernetic backbone, you bronze asshat? I fucking dare you. I dare you to step to me. Yeah, I thought so, you know what special "utensils" I got up in this bitch.
THREEPIO: You watch your language!
...on the Death Star...
LUKE: Who do you think...
Suddenly Artoo begins to whistle and beep a blue streak. Luke goes over to him.
LUKE: What is it?
THREEPIO: I'm afraid I'm not quite sure, sir. He says "I found her", and keeps repeating, "She's here."
LUKE: Well, who...who has he found?
Artoo whistles a frantic reply.
R2-D2: OMGOMGOMGOMGOMGOMG IFOUNDHERIFOUNDHER SHE'SHERESHE'S HERE AND VADER'S TOTALLY KEEPING HER IN HER UNDERWEAR I AM SERIOUSLY RECORDING THIS SECURITY FOOTAGE OMGOMGOMG IFOUNDHERIFOUNDHER
THREEPIO: Princess Leia.
INTERIOR: MILLENNIUM FALCON, CENTRAL HOLD AREA. Luke looks downward sadly, shaking his head back and forth, as the princess smiles comfortingly at him.
LUKE: I can't believe he's gone.
R2-D2: I dunno, I mean it was about fucking time.
LEIA: There wasn't anything you could have done.
GOLD LEADER: Pardon me for asking, sir, but what good are snub fighters going to be against that?
DODONNA: Well, the Empire doesn't consider a small one-man fighter to be any threat, or they'd have a tighter defense. An analysis of the plans provided by Princess Leia has demonstrated a weakness in the battle station.
Artoo-Detoo stands next to a similar robot, makes beeping sounds, and turns his head from right to left.
R2-D2: Yeah, that's right. Who brought home the motherfucking plans? This guy. Who's your daddy now, bitches?
A large burst of Vader's laserfire engulfs Artoo. The arms go limp on the smoking little droid as he makes a high-pitched sound. Smoke billows out around little Artoo and sparks begin to fly.
LUKE: I've lost Artoo!
Artoo's beeping sounds die out.
R2-D2: FUCKING A YOU COCK-NOMMING MOTHERfucking Skywa..lker...s..... ha..te..youall....
It works in the other movies too, of course. For instance, Empire:
DERLIN: Your Highness, there's nothing more we can do tonight. The shield doors must be closed.
Chewie lets out a long, mournful howl, somewhat like a coyote. Artoo begins a complex series of efficient beeps.
R2-D2: Those fucking dipshits, now they're both out there. I swear to god if I had one credit for every time a Skywalker did something retarded I'd have... well hell, let's compute it. What is it, 775 times, now...?
THREEPIO: Artoo says the chances of survival are seven hundred seventy-five...to one.
And of course, Jedi:
INT SAIL BARGE, OBSERVATION DECK. Leia is struggling against her chains in desperation as Artoo zips through the tumult of confused monsters to the rescue; the stubby little droid extends a small laser gun and blasts the chain apart.
R2-D2: Oh hells to the yeah, baby. You can come be my slave anytime.
You will never be able to watch Star Wars the same way again.